> Oddly, I gather rats cannot barf, which is why coumarin works as rat killer.

I think your recollection is off by a little. Coumarin is an anticoagulant and damages rats' livers: https://en.wikipedia.org/wiki/Coumarin

You might be thinking of corn gluten meal, which kills rats by preventing them from absorbing water? It's sold as RatX: https://www.ecoclearproducts.com/pages/ratx-faqs